Acts 18:18

Shorn, &c. It was customary among the Jews to make vows of abstaining from all inebriating liquor, not to cut their hair for a limited time, &c. This was the vow of the Nazarites, mentioned in Numbers vi. 18; Acts xxii. 24. St. Paul had probably taken upon himself some obligation of this kind; perhaps in condescension to the Jews, who were yet weak in faith. The time being now expired, he cut his hair as before. It was lawful for converts to observe these legal ceremonies, till the gospel was perfectly established, provided they did not place their hopes of salvation in them, or believe that the faith and grace of Christ were ineffectual without them. (Denis the Carthusian) --- For he had a vow, that is, Paul, not Aquila. This seems to have been such a vow, as those called Nazarenes, used to make, of abstaining from wine for a time, of not cutting their hair, and of making some offerings in the temple at Jerusalem. (Witham)